Added `"sideEffects": false` to `package.json`

[neoncube2]

What is the purpose of this pull request? (put an "X" next to an item)

• [ ] Documentation update
• [ ] Bug fix
• [ ] Feature
• [ ] Code style update (formatting)
• [ ] Refactoring (no functional changes)
• [ ] CI related changes
• [X] Other, please explain:

Added "sideEffects": false to package.json. Apparently, this can enable more tree-shaking, in some cases: documentation

Other popular libraries, such as axios, have been adding this flag: https://github.com/axios/axios/pull/5025

What changes did you make? (Give an overview)

See above

Is there anything you'd like reviewers to focus on?

It'd probably be smart for someone with a project that is directly using mitt to test this change.

Does this PR introduce a breaking change? (What changes might other developers need to make in their application due to this PR?)

Shouldn't be a breaking change